Drying lumber was conventionally performed in a batch kiln process, where an insulated chamber is used that is adapted to control several drying process conditions, including, but not limited to air temperature in the kiln, air speed across the lumber, and the relative humidity in the chamber. As these kilns are a closed atmosphere, packages of sawn lumber, often referred to as green lumber, separated by stickers are placed in the kiln in batches. The packages are often loaded vertically, horizontally, and end to end.
Once the batch of packages are in place, the chamber is closed and a schedule or recipe of temperatures and relative humidity is initiated for a determined time interval or until a certain moisture content in the lumber is achieved. Generally, the schedule gradually increases the temperature in the chamber and lowers the relative humidity. This allows the lumber to release its moisture to the surrounding air, which may then be vented to the outside atmosphere.
The particular schedule used and the drying time varies depending on a number of factors, including, but not limited to, lumber type/species, thickness, moisture content, end use of the lumber and the like. Once the schedule has run, the kiln doors are opened and the packages are removed from the kiln chamber and further prepared for shipping to a final destination. This opens the chamber to atmospheric conditions and can often require a significant amount of time and energy to bring the next charge of green lumber up to drying conditions. In a batch drying process, as described above, ordinary maintenance could and would occur between batches.
Recently, kiln technology has introduced the Dual Path or continuous Kiln, U.S. Pat. No. 7,963,048 (Pollard), the complete disclosure of which is incorporated herein by reference. A dual path kiln is one that includes a continuous kiln having one or more chambers and at least two lumber charge paths adapted to convey lumber through the kiln in opposite directions. The lumber is pushed at a given rate of speed in a counter-flow process through the kiln in a continuous 24/7 process. Ordinary maintenance is difficult, if not impossible as it is very inefficient, time consuming and cost prohibitive to shut down this drying process.
One of the components of a kiln is a motorized fan(s) that is used to transfer the heat (either steam or direct fired) through the kiln. Depending on the size of the kiln, several fans are used. These fans are mounted on shafts and bearings, which are located inside the drying chamber, in direct contact with the heat and moisture. This atmosphere is very caustic and acidic and causes fan bearing failure problems prematurely, prior to scheduled maintenance times.
These fans are typically mounted on individual shafts with the shafts protruding through the kiln sidewall. These shafts can have two or more bearings located between the electric motor and fan assembly. Heat causes expansion problems in the drive shafts, which introduce unwanted stresses into the bearings, contributing to their failure. Alignment of these shafts and bearings is critical and changes with temperature, making it difficult to obtain long bearing life. Since the drive shaft is typically solid, natural frequency problems often occur, causing premature bearing failure. A conventional kiln fan is shown in FIG. 1 (prior art), having a solid shaft 6 connecting the fan blade 2 to the motor 10, and being held in place by bearings 4. The shaft 6 penetrates through the kiln wall 12.